Hair straightening or relaxing has become increasingly popular in view of hair styles which require relatively or perfectly straight hair. Several types of chemical hair straightening products are available. But strong alkali, especially sodium hydroxide, is generally considered the most effective agent for substantially permanently straightening kinky-curly negroid hair.
In order to achieve maximal straightening, however, relatively high concentrations of greater than about 2 weight percent sodium hydroxide are generally applied to and maintained in contact with the hair for a relatively long time. This process results in decreased hair condition benefits and increased skin irritation.
Hair straightening or relaxer products most commonly used in salons and in the home contain as the sole active hair-straightening agent either a strong alkali, such as sodium hydroxide or guanidine hydroxide; a sulfite, typically an ammoniacal mixture of bisulfite and sulfite; or a thiol compound, typically ammonium thioglycolate. All of these types of products exert their primary reducing effect by breaking the cystine disulfide bonds present in hair keratin, referred to as hair keratin-disulfide bonds. The chemical action of alkali-based straightening, sulfite-based straightening and thiol-based straightening is known to differ to varying extents. Alkali, in particular, produces additional stable crosslinks in the hair that are not normally present in virgin hair.
The majority of hair-straightening kits sold for home use are based on the sulfite or thioglycolate straighteners. However, these have several inherent disadvantages. One major disadvantage is the highly offensive odor of the thioglycolate solutions and of the thiol-reduced hair. Another disadvantage is that thiol-based straightening requires the use of an oxidizing neutralizer, such as hydrogen peroxide, to chemically relink the hair keratin-disulfide bonds and to stop the straightening process quickly. Since the thiol-reduced hair is in an alkaline state, any excess neutralizer must also be removed to avoid bleaching the natural color of the hair.
Sulfite-based straighteners have similar disadvantages. For example, sulfite-containing solutions can deteriorate gradually and release offensive odors of sulfur dioxide. Sulfite-reduced hair also must be neutralized by bringing the hair to an alkaline pH to reverse the keratin-sulfite reaction and to chemically re-link the hair keratin-disulfide bonds.
On the other hand, strong alkalis, such as sodium hydroxide and guanidine hydroxide, have several advantages over the sulfite or thioglycolate agents. These alkalis do not have a highly objectionable odor or cause such an odor on reducing the hair. Alkali-straightened hair is treated at a highly alkaline pH of between above 12 and about 14. At that alkalinity, alkalis are known to form stable, irreversible crosslinks of lanthionine and lysinoalanine in the reduced hair making a chemical re-linking step unnecessary. Thus, the only step required following an alkali-based straightening process is to remove substantially all excess alkaline solution to avoid and minimize damage to the hair protein or skin. For this purpose, an acidic shampoo is usually used to neutralize residual alkali on and remove it from the hair and scalp. Guanidine hydroxide, unlike sodium hydroxide, is not chemically stable in solution for any practical storage period. Consequently, it must be freshly prepared prior to using (usually within 24 hours). For this reason, guanidine hydroxide-based straighteners are supplied as a two-component package. Thus, despite its causticity, sodium hydroxide-based hair straightening or relaxer kits, which became available around 1958, are still popular and widely used in professional shops. The sodium hydroxide-based relaxer has also continuously gained popularity for home use since its introduction to the retail market in 1971.
The main advantage of a sodium hydroxide straightener is relatively good straightening of naturally kinky-curly negroid hair. Additionally, the straightening effect is more permanent; i.e., less likely to revert to a curly state after shampooing and wearing than is hair straightened with other straighteners.
However, the sodium hydroxide must remain in contact with kinky-curly hair long enough to allow sufficient irreversible crosslinks to form rather than reversible temporary salt bond linkages. Otherwise, only a loosened wavy curl pattern is achieved, an effect sometimes called texturizing, rather than permanent straightening. Thus, in order to permanently straighten relatively resistant coarse hair having a natural kinky or tight curl pattern, a process time of about 20 minutes or longer ordinarily would be required.
The principal disadvantage of sodium hydroxide-based hair straighteners, of course, is their causticity which can adversely affect hair condition, leaving it in a brittle state and harsh to the touch. Consequently, prolonged or unnecessary exposure of hair to strong alkali at above pH 12 can weaken, break and even dissolve the hair. In the interest of avoiding loss of hair integrity, relatively short process times of about 18 minutes or less are typically employed, thereby sacrificing some measure of permanency. Thus, it would be desirable to initiate the formation of permanent linkages in the cortex of the hair to boost permanent straightening effects within such a practical short process.
The concentration of sodium hydroxide used for modern hair-straightening procedures can vary between about 1.5 to about 3.5 weight percent, depending on whether the product is a "base" type or "no-base" type relaxer. The term "base type relaxer" means that the scalp and hair line must be coated with a protective oleaginous base, such as petrolatum, mineral oil and lanolin, before applying the hair relaxer. The term "no-base type relaxer" means that the scalp need not be coated with a protective base. In some cases, where the no-base type hair relaxer incorporates sodium hydroxide in an oleaginous cream base, a protective base frequently need only be applied to the hairline to protect the skin around the forehead, ears and neckline. No-base type relaxer processes, therefore, are preferred.
Some past attempts have been made to use chemical straighteners, other than sodium hydroxide, sulfite and thioglycolates, such as mercapto-substituted compounds and various combinations of chemical treatments and heat. A discussion of these studies can be found in the books by deNavarre, The Chemistry and Manufacture of Cosmetics, Second Edition, Vol. IV, Continental Press, Orlando, Fl. (1975) and by Sagarin, Cosmetics: Science and Technology, Second Edition, Vol. 2, Wiley-Interscience, New York, NY (1972). The pertinent disclosures of both of these books are incorporated herein by reference. A review of other chemical hair straighteners and waving agents reported in the patent literature also can be found in Cosmetics & Toiletries, 94, 61-69 (April 1979) and 100, 23-29 (April 1985), also incorporated herein by reference. These reported attempts have met with varying degrees of success. But, except for possibly guanidine hydroxide, other chemical hair-straightening agents have not achieved any substantial measure of practical or commercial importance in the hair-straightening arts beyond scientific interest.
In the practice of cold-waving, which by analogy extends to the practice of hair-straightening, thiol compounds that are effective in a cold-wave process are generally used only at pH values below about 10. The terms "cold-wave", "cold-waving" and "cold-wave process" are used in the conventional sense to mean that permanent hair waving or hair straightening is achieved at between ambient room temperature and body temperature, usually at about 30 degrees C (about 86 degrees F) without the assistance of externally applied heat.
The terms "hair-straightening", "hair-relaxing" and grammatical variations thereof will be used interchangeably herein to denote the removal of natural curl from naturally tight curly or wavy hair. The term "permanent hair-straightening" denotes the removal of substantially all curliness to a visibly straight configuration, as opposed to "texturizing hair-straightening" which denotes a loosened wave pattern.
Cosmetically useful thiols in cold wave processes typically have a relatively low molecular weight of below about 140, and are monofunctional, alpha- or beta-mercapto substituted, carboxylic acids having from about 2 to about 5 carbon atoms. However, these thiol compounds are generally not useful for straightening hair at a pH above about 11.5. At above about pH 12, for example, alkali metal salts of thioglycolic acid and of thiolactic acid are known to weaken and dissolve (depilate) hair within a process time of less than about 15 minutes.
Thus, the effective keratin-disulfide reducing action of the foregoing thiols normally cannot be safely combined with alkali to initiate and boost the permanent straightening effects of alkali-based hair-straighteners.
In the interest of maintaining cosmetically safe alkali-based hair straightening, it is desirable to use relatively low levels of sodium hydroxide of between about 1.5 to about 2 weight percent. However, it is necessary to maintain a desirable alkalinity of above pH 12, preferably between about pH 12.5 and about pH 13.5, in order to achieve maximal permanent hair straightening. It is known, for example, that sodium hydroxide at below about pH 12 and at a titratable alkalinity of less than about 0.5 weight percent provides substantially no straightening of curly hair.
Additionally, relatively low levels of alkalinity in combination with short contact times can only form reversible salt linkages in the hair. As a consequence, instead of achieving effective permanent hair-straightening, the caustic action tends to be confined to the surface boundary or cuticle of the hair. This disadvantage further causes a harsher hair condition.
In some instances, strong alkali discolors the natural color of the hair. For example, the tone of natural brown hair is reddened and natural white or grey hair is undesirably yellowed and brightness is dulled. Thus, another disadvantage is delustering of the natural sheen of the hair.
Some strides have been made in improving the condition of sodium hydroxide-straightened hair by incorporating conditioners into the alkaline product. See, for example, U.S. Pat. No. 4,175,572, which issued to the present assignee and which is incorporated herein by reference.
There is still a need for boosting the permanent hair-straightening effects obtained with compositions having relatively low concentrations of sodium hydroxide in a no-base type hair-relaxer process to achieve permanent hair straightening effects equivalent to or greater than those obtained from higher alkali amounts presently used. An ideal sodium hydroxide-based hair straightening composition, system and method would initiate keratin disulfide reduction sufficiently to boost the permanent straightening action of strong alkali in achieving the foregoing benefits within a relatively fast processing time, yet provide good hair condition.